How do I remove duplicates from a C array

Question

I have been working with a string   array in C  that gets returned from a function call  I could possibly cast to a Generic collection  but I was wondering if there was a better way to do it  possibly by using a temp array   What is the best way to remove duplicates from a C  array

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This code 100  remove duplicate values from an array as I used a i       You can convert it in any OO language          for int i 0 i lt size i          for int j i 1 j lt size j                  if a i     a j                         for int k j k lt size k                                   a k  a k 1                             j--              size--

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-- This is Interview Question asked every time  Now i done its coding   static void Main string   args                    int   array   new int     4  8  4  1  1  4  8                            int numDups   0  prevIndex   0               for  int i   0  i  lt  array Length  i                                  bool foundDup   false                  for  int j   0  j  lt  i  j                                          if  array i     array j                                                 foundDup   true                          numDups       Increment means Count for Duplicate found in array                          break                                                                               if  foundDup    false                                        array prevIndex    array i                       prevIndex                                                    Just Duplicate records replce by zero              for  int k   1  k  lt   numDups  k                                                 array array Length - k      0                                            Console WriteLine  Console program for Remove duplicates from array                 Console Read

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int size   a Length          for  int i   0  i  lt  size  i                          for  int j   i   1  j  lt  size  j                                  if  a i     a j                                         for  int k   j  k  lt  size  k                                                  if  k    size - 1                                                        int temp   a k                               a k    a k   1                               a k   1    temp                                                                       j--                      size--

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Add all the strings to a dictionary and get the Keys property afterwards  This will produce each unique string  but not necessarily in the same order your original input had them in   If you require the end result to have the same order as the original input  when you consider the first occurance of each string  use the following algorithm instead    Have a list  final output  and a dictionary  to check for duplicates  For each string in the input  check if it exists in the dictionary already If not  add it both to the dictionary and to the list   At the end  the list contains the first occurance of each unique string   Make sure you consider things like culture and such when constructing your dictionary  to make sure you handle duplicates with accented letters correctly

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Here is a O n n  approach that uses O 1  space    void removeDuplicates char  strIn        int numDups   0  prevIndex   0      if NULL    strIn  amp  amp   strIn      0                 int len   strlen strIn           for int i   0  i  lt  len  i                          bool foundDup   false              for int j   0  j  lt  i  j                                  if strIn j     strIn i                                         foundDup   true                      numDups                        break                                               if foundDup    false                                strIn prevIndex    strIn i                   prevIndex                                     strIn len-numDups      0             The hash linq approaches above are what you would generally use in real life  However in interviews they usually want to put some constraints e g  constant space which rules out hash or no internal api - which rules out using LINQ

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Simple solution   using System Linq       public static int   Distinct int   handles        return handles ToList   Distinct   ToArray

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The best way  Hard to say  the HashSet approach looks fast  but  depending on the data  using a sort algorithm  CountSort    can be much faster  using System  using System Collections Generic  using System Linq  class Program       static void Main                 Random r   new Random 0   int   a  b   new int 1000000           for  int i   b Length - 1  i  gt   0  i--  b i    r Next b Length           a   new int b Length   Array Copy b  a  b Length           a   dedup0 a   Console WriteLine a Length           a   new int b Length   Array Copy b  a  b Length           var w   System Diagnostics Stopwatch StartNew            a   dedup0 a   Console WriteLine w Elapsed   Console Read               static int   dedup0 int   a      48 ms                 return new HashSet lt int gt  a  ToArray               static int   dedup1 int   a      68 ms               Array Sort a   int i   0  j   1  k   a Length  if  k  lt  2  return a          while  j  lt  k  if  a i     a j   j    else a   i    a j             Array Resize ref a  i   1   return a             static int   dedup2 int   a       8 ms               var b   new byte a Length   int c   0          for  int i   0  i  lt  a Length  i                 if  b a i      0    b a i     1  c              a   new int c           for  int j   0  i   0  i  lt  b Length  i    if  b i   gt  0  a j      i          return a           Almost branch free  How  Debug mode  Step Into  F11  with a small array   1 3 1 1 0      static int   dedupf int   a       4 ms               if  a Length  lt  2  return a          var b   new byte a Length   int c   0  bi  ai  i  j          for  i   0  i  lt  a Length  i              ai   a i   bi   1   b ai   b ai      byte bi  c    bi            a   new int c   i   0  while  b i     0  i    a 0    i            for  j   0  i  lt  b Length  i    a j    bi   b i      bi   i  return a         A solution with two nested loops might take some time  especially for larger arrays      static int   dedup int   a                int i  j  k   a Length - 1          for  i   0  i  lt  k  i                for  j   i   1  j  lt   k  j    if  a i     a j   a j--    a k--           Array Resize ref a  k   1   return a

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You could possibly use a LINQ query to do this   int   s     1  2  3  3  4   int   q   s Distinct   ToArray

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The following piece of code attempts to remove duplicates from an ArrayList though this is not an optimal solution  I was asked this question during an interview to remove duplicates through recursion  and without using a second temp arraylist     private void RemoveDuplicate       ArrayList dataArray   new ArrayList 5                dataArray Add  1                dataArray Add  1                dataArray Add  6                dataArray Add  6                dataArray Add  6                dataArray Add  3                dataArray Add  6                dataArray Add  4                dataArray Add  5                dataArray Add  4                dataArray Add  1                 dataArray Sort                 GetDistinctArrayList dataArray  0      private void GetDistinctArrayList ArrayList arr  int idx                  int count   0               if  idx  gt   arr Count  return               string val   arr idx  ToString                foreach  String s in arr                                if  s Equals arr idx                                          count                                                 if  count  gt  1                                arr Remove val                   GetDistinctArrayList arr  idx                             else                               idx    1                  GetDistinctArrayList arr  idx

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Removing duplicate and ignore case sensitive using Distinct  amp  StringComparer InvariantCultureIgnoreCase string   array   new string      quot A quot    quot a quot    quot b quot    quot B quot    quot a quot    quot C quot    quot c quot    quot C quot    quot A quot    quot 1 quot     var r   array Distinct StringComparer InvariantCultureIgnoreCase  ToList    Console WriteLine r Count      return 4 items

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If you needed to sort it  then you could implement a sort that also removes duplicates   Kills two birds with one stone  then

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This might depend on how much you want to engineer the solution - if the array is never going to be that big and you don t care about sorting the list you might want to try something similar to the following       public string   RemoveDuplicates string   myList            System Collections ArrayList newList   new System Collections ArrayList             foreach  string str in myList              if   newList Contains str                   newList Add str           return  string   newList ToArray typeof string

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Find answer below  class Program       static void Main string   args                var nums   new int     1  4  3  3  3  5  5  7  7  7  7  9  9  9            var result   removeDuplicates nums           foreach  var item in result                        Console WriteLine item                       static int   removeDuplicates int   nums                nums   nums ToList   OrderBy c   gt  c  ToArray            int j   1          int i   0          int stop   0          while  j  lt  nums Length                        if  nums i     nums j                                 nums i   1    nums j                   stop   i   2                  i                              j                      nums   nums Take stop  ToArray            return nums           Just a bit of contribution based on a test i just solved  maybe helpful and open to improvement by other top contributors here  Here are the things i did   I used OrderBy which allows me order or sort the items from smallest to the highest using LINQ I then convert it to back to an array and then re-assign it back to the primary datasource So i then initialize j which is my right hand side of the array to be 1 and i which is my left hand side of the array to be 0  i also initialize where i would i to stop to be 0  I used a while loop to increment through the array by going from one position to the other left to right  for each increment the stop position is the current value of i   2 which i will use later to truncate the duplicates from the array  I then increment by moving from left to right from the if statement and from right to right outside of the if statement until i iterate through the entire values of the array  I then pick from the first element to the stop position which becomes the last i index plus 2  that way i am able to remove all the duplicate items from the int array  which is then reassigned

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The following tested and working code will remove duplicates from an array  You must include the System Collections namespace   string   sArray     a    b    b    c    c    d    e    f    f    var sList   new ArrayList     for  int i   0  i  lt  sArray Length  i          if  sList Contains sArray i      false            sList Add sArray i             var sNew   sList ToArray     for  int i   0  i  lt  sNew Length  i          Console Write sNew i        You could wrap this up into a function if you wanted to

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Here is the HashSet lt string gt  approach   public static string   RemoveDuplicates string   s        HashSet lt string gt  set   new HashSet lt string gt  s       string   result   new string set Count       set CopyTo result       return result      Unfortunately this solution also requires  NET framework 3 5 or later as HashSet was not added until that version  You could also use array Distinct    which is a feature of LINQ

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private static string   distinct string   inputArray                        bool alreadyExists              string   outputArray   new string                    for  int i   0  i  lt  inputArray Length  i                                  alreadyExists   false                  for  int j   0  j  lt  outputArray Length  j                                          if  inputArray i     outputArray j                           alreadyExists   true                                            if  alreadyExists  false                                                        Array Resize lt string gt  ref outputArray  outputArray Length   1                               outputArray outputArray Length-1    inputArray i                                                       return outputArray

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NOTE   NOT tested   string   test string   myStringArray        List lt String gt  myStringList   new List lt string gt         foreach  string s in myStringArray                if   myStringList Contains s                         myStringList Add s                       return myStringList ToString        Might do what you need     EDIT Argh    beaten to it by rob by under a minute

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Generic Extension method    public static IEnumerable lt TSource gt  Distinct lt TSource gt  this IEnumerable lt TSource gt  source  IEqualityComparer lt TSource gt  comparer        if  source    null          throw new ArgumentNullException nameof source         HashSet lt TSource gt  set   new HashSet lt TSource gt  comparer       foreach  TSource item in source                if  set Add item                         yield return item

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protected void Page Load object sender  EventArgs e        string a    a b c d e v       string   b   a Split           string   c   b Distinct   ToArray         if  b Length    c Length                for  int i   0  i  lt  b Length  i                          try                               if  b i  ToString      c i  ToString                                          Response Write  Found duplicate     b i  ToString                         return                                              catch  Exception ex                                Response Write  Found duplicate     b i  ToString                     return                                                  else               Response Write  No duplicate

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List lt String gt  myStringList   new List lt string gt     foreach  string s in myStringArray        if   myStringList Contains s                 myStringList Add s              This is O n 2   which won t matter for a short list which is going to be stuffed into a combo  but could be rapidly be a problem on a big collection

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using System  using System Collections Generic  using System Linq    namespace Rextester       public class Program               public static void Main string   args                         List lt int gt  listofint1   new List lt int gt    4  8  4  1  1  4  8               List lt int gt  updatedlist  removeduplicate listofint1               foreach int num in updatedlist                 Console WriteLine num                       public static List lt int gt  removeduplicate List lt int gt  listofint                          List lt int gt  listofintwithoutduplicate  new List lt int gt                     foreach var num in listofint                                       if  listofintwithoutduplicate Any p  gt p  num                                                       listofintwithoutduplicate Add num                                                              return listofintwithoutduplicate

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Below is an simple logic in java you traverse elements of array twice and if you see any same element you assign zero to it plus you don t touch the index of element you are comparing   import java util    class removeDuplicate  int    y    public removeDuplicate int   array       y array       for int b 0 b lt y length b             int temp   y b           for int v 0 v lt y length v                 if  b  v  amp  amp  temp  y v                    y v  0

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you can using This code when work with an ArrayList  ArrayList arrayList    Add some Members    arrayList Add  ali    arrayList Add  hadi    arrayList Add  ali       Remove duplicates from array   for  int i   0  i  lt  arrayList Count  i                 for  int j   i   1  j  lt  arrayList Count   j               if  arrayList i  ToString      arrayList j  ToString                     arrayList Remove arrayList j

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public static int RemoveDuplicates ref int   array        int size   array Length          if 0 or 1  return 0 or 1      if  size   lt  2            return size             int current   0      for  int candidate   1  candidate  lt  size    candidate            if  array current     array candidate                 array   current    array candidate                           index to count conversion      return   current

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Maybe hashset which do  not  store  duplicate  elements  and  silently  ignore  requests  to  add duplicates   static void Main         string textWithDuplicates    aaabbcccggg             Console WriteLine textWithDuplicates Count           var letters   new HashSet lt char gt  textWithDuplicates       Console WriteLine letters Count          foreach  char c in letters  Console Write c       Console WriteLine           int   array   new int     12  1  1  1  1  1  1  1  2  2  2  2  2         Console WriteLine array Count         var distinctArray   new HashSet lt int gt  array       Console WriteLine distinctArray Count          foreach  int i in distinctArray  Console Write i

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strINvalues    1 1 2 2 3 3 4 4   strINvalues   string Join      strINvalues  Split      Distinct   ToArray     Debug Writeline strINvalues     Kkk Not sure if this is witchcraft or just beautiful code  1 strINvalues  Split      Distinct   ToArray    2 string Join      XXX    1 Splitting the array and using Distinct  LINQ  to remove duplicates 2 Joining it back without the duplicates   Sorry I never read the text on StackOverFlow just the code  it make more sense than the text

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Tested the below  amp  it works  What s cool is that it does a culture sensitive search too  class RemoveDuplicatesInString       public static String RemoveDups String origString                String outString   null          int readIndex   0          CompareInfo ci   CultureInfo CurrentCulture CompareInfo            if String IsNullOrEmpty origString                         return outString                     foreach  var ch in origString                        if  readIndex    0                                outString   String Concat ch                   readIndex                    continue                             if  ci IndexOf origString  ch ToString   ToLower    0  readIndex     -1                                  Unique char as this char wasn t found earlier                  outString   String Concat outString  ch                                                 readIndex                         return outString              static void Main string   args                String inputString    aAbcefc           String outputString           outputString   RemoveDups inputString            Console WriteLine outputString              --AptSenSDET

[c#] How do I remove duplicates from a C# array?

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